Process for preparation of magnetic alloy powder

ABSTRACT

A crystalline oxalate containing Al in addition to usual Fe, Co and/or Ni in an amount about 1-20 mole% of the total metals is precipitated from a mixed aqueous solution of respective metal chlorides. The Al-containing oxalate gives a magnetic alloy powder having a large coercive force and high residual magnetization through reduction by H2 at a temperature between 300* and 400*C.

United States Patent 1191 Takahashi et a1.

[ PROCESS FOR PREPARATION OF MAGNETIC ALLOY POWDER [75] Inventors:Noboru Takahashi; Mutsuaki Nakamura; Sadao Ozaki; Hiromasa lsono; Iwao Fukushima, all of Yokohama, Japan [73] Assignee: Victor Company of Japan,Limited,

Japan [22] Filed: Apr. 29, 1974 [21] Appl. No.: 465,300

[58] Field of Search 75/.5 A, .5 AA, .5 R;'

[56] References Cited UNITED STATES PATENTS 2,497,268 2/1950 Neel 75/ 5AA 1 Dec. 9, 1975 2,651,105 9/1953 Neel 75/.5 AA 2,660,522 11/1953 LaTronche 75/.5 AA 2,853,374 9/1958 Schaufelberger 75/.5 AA

OTHER PUBLICATIONS Bozorth, R; Ferro Magnetism, New York, 1951. pp.184-185.

Primary Examiner-Walter R. Satterfield Attorney, Agent, or Firm-RobertE. Burns; Emmanuel J. Lobato; Bruce L. Adams [57] ABSTRACT 10 Claims, 17Drawing Figures US. Patent Dec. 9, 1975 Sheet 1 of2 3,925,114

FIG. 5

20 Al (mole '1.)

FIG. I

PRIOR ART 300 A00 TEMPERATURE ("c m 63,; mzbjmmv women 5280 FIG. 6

10 Al (mole'h) FIG. 7

Al (molc'h) FIG. 3

PRIOR ART TEMPERATURE c) FIG. 8

10 Al (moleh) FIG. 4

PRIOR ART TEMPERATURE W2) U.S. Patent Dec. 9, 1975 Sheet 2 of 23,925,114

FIG. I3

300 300 TEMPERATURE c) F I G 9 300 400 TEMPERATURE (c) A w3 M2255 womomM2058 FIG I4 300 400 TEMPERATURE c) FIG. I O

300 400 TEMPERATURE c) FIG. I6

300 400 TEMPERATURE c) A mAm MESH: 5:35.22: 2398 FIG. I I

300 A00 TEMPERATURE (c) F I G I7 300 400 TEMPERATURE ("c PROCESS FORPREPARATION OF MAGNETIC ALLOY POWDER The present invention relatesgenerally to a process for the preparation of magnetic powder materialscomposed of Fe, Co and/or Ni, and more particularly to an improvement ofa process of preparing such magnetic alloy powders by reduction ofoxalates at elevated temperatures.

A magnetic material for use in recording media such as magnetic tapes isrequired to have a high residual magnetization and a large coerciveforce. In recent years, the familiar iron oxide powder has beengradually replaced by superior magnetic metal or alloy powders to meetgrowing requirements for magnetic tapes of higher recording densities.Typical examples of such magnetic powder materials are ferromagneticmetals such as Fe, Co and Ni, and their alloys.

These magnetic metal and alloy powders are usually prepared by areduction process, in which a crystalline powder of a single, binary orternary oxalate (or formate) is reduced through heating in a reducingatmosphere. As is known, chemical and physical properties such ascomposition, mean particle size and particle size distribution of amagnetic powder material produced by such a process are dependent onreaction conditions used for the preparation of the oxalate or formate.It is also known that the magnetic properties of the obtain ed powdermaterials are greatly dependent of the reduction reaction conditionsbesides the above reaction conditions. In general, easiness of reductionof these oxalates and extent of sintering of unit particles are variedas the reduction reaction conditions are varied. Until now, therefore,various reduction temperatures and heating times have been tried toobtain an alloy powder of excellent and balanced magnetic properties.

However, a problem in the art is that such reduction conditions have notsimilar but rather diverse influences on the various magnetic propertiesof a magnetic alloy powder. For example, a set of conditions suitablefor increasing the saturation magnetization causes the unit particles togrow excessively large. This is due to progress of sintering, andresults in a decreased coercive force. Under another set of conditions,which gives a large coercive force, saturation magnetization is greatlyreduced because of insufficient progress of the reduction reactionand/or a partial oxidation of the reduced powder due to its relativelystrong chemical activity. Consequently, it has been next to impossibleto obtain a magnetic powder material having both a high residualmagnetization value and a large coercive force, no matter what set ofreduction conditions might have be chosen.

It is an object of this invention to provide such powders.

It is therefore a general object of the present invention to provide aprocess of preparing a magnetic alloy powder having a high residualmagnetization and a large coercive force with an extremely littlesacrifice in satu ration magnetization.

It is a more particular object of the invention to pro' vide a processof preparing an intermediate oxalate which gives such a magnetic powdermaterial upon reduction.

It is another object of the invention to provide an appropriatetemperature range for reduction of such an oxalate.

According to the present invention, these objects are accomplishedessentially by allowing Fe, Co and/or Ni ions to react with oxalic acid,in the presence of Al ion, to precipitate a crystalline oxalate powdercontaining Al and at least one of the above three ferromagnetic metals.The amount of Al is preferably from I to 20 mole% of the total metalmoles. The obtained crystalline oxalate powder is then reduced into amagnetic alloy powder in the usual way preferably at a temperature rangebetween about 300to about 400C.

The invention will be more clearly understood from the followingdetailed description with reference to the accompanying drawings, inwhich:

FIGS. 1-4 are graphs showing variations of magnetic properties of amagnetic alloy powder prepared by a conventional process as function ofreduction temperatures;

FIGS. 5-8 are graphs showing variations of similar properties of amagnetic alloy powder prepared by the process of the invention, asfunction of Al content;

FIGS. 9-12 are graphs similar to FIGS. 1-4 for a magnetic alloy powderprepared by the process of the invention with 4 mole% Al; and

FIGS. 13-17 are similar graphs to FIGS. 9-12 in the case of 7 mole% Al.

As summarized above, the improvement according to the invention, by theintroduction of Al into a starting solution is adaptable for thepreparation of either any individual metal powder of Fe, Co and Ni withaluminum or any of their mixed alloys. Hereinafter, however, thedescription will concern only the preparation of Fe- Co-Ni seriesternary magnetic alloys with aluminum as a typical example to facilitatecomparison of experimental data.

In a process according to the invention, water-soluble salts such aschlorides of Fe, Co and Ni are dissolved in water together with asimilar salt of Al. The aqueous solution is adjusted to a suitableacidity, preferably to about pI-I L5, and is then mixed and stirred witha solution of oxalic acid C I-I O, 2H O in a water miscible solvent,preferably in acetone containing a small amount of toluene. The stirringis continued usually for about 10 min at room temperature or at aslightly elevated temperature to precipitate crystalline multicomponentFe, Co, Ni and Al oxalates. The precipitate is considered not a simplemixture of four kinds of oxalates, but a crystalline four-componentoxalate. A crystalline powder of this oxalate obtained throughfiltration, washing and drying of the precipitate is heated for severalhours in a reducing atmosphere, usually in a stream of hydrogen, at atemperature ranging from about 300 to 400C to give a novel magneticalloy powder. To accomplish the objects of the invention, it isnecessary to establish a substantial co-existence of Al ion with theother three metal ions; in other words, the amount of Al in the aqueoussolution should be at least about 1 mole% ofthe total metals. Existenceof only a trace of Al, for example, up to 0.] mole% has little effect onthe magnetic properties of a resulting alloy powder.

The introduction of Al into the intermediate oxalate crystal systemcontributes to remarkably augment both,

EXAMPLE l An oxalic acid solution was prepared by dissolving 252g of Cl-l O .2H O in 200 ml of acetone followed by the addition of 5 ml oftoluene. The temperature of the solution was kept at 35C in a constanttemperature bath. This kind of oxalic acid solutions were usedthroughout the succeeding examples.

Meanwhile, 8.8g of FeCl .nH O, 8.6g of CoCl .6H O, 0.4g of NiCl .6l-l Oand 0.2g of AlCl .6l-l O were dis solved in 75 ml of water. The aqueoussolution was adjusted to pH L5 and was heated to 35C.

Then, the aqueous solution was poured into the oxalic acid solution allat once with vigorous stirring, and stirring was continued for min,keeping the liquid temperature at 35C, to allow the metal ions in thesolution to completely react with the acid. The resulting precipitatewas filtered, washed and dried to give a crystalline powder of mixed Fe,Co, Ni and Al oxalates. This procedure for preparation of the oxalateswas unchanged throughout the succeeding examples.

The obtained oxalate powder was reduced into an alloy powder under thefollowing conditions, which where common to all examples except fortemperature.

80 mg of the oxalate powder was put in a platinum boat and was heated ina 0.5 I/min stream of hydrogen at 310C for 6 hr.

The resulting alloy powder showed the following magnetic properties:

Coercive force 880 Oe Saturation magnetization l9] emy/g Residualmagnetization 64 emu/g These values and those which were obtained in thefollowing examples and references are shown together in a Table at theend of the Reference example 2.

EXAMPLE 3 Example 2 was repeated except that the reduction was carriedout at 330C.

EXAMPLE 4 Example 3 was repeated except that the reduction temperaturewas 370C.

EXAMPLE 5 The chloride solution was prepared by dissolving 8.2g of FeCl.nH O, 8.0g of CoCl .6H O, 0.3g of NiCl- -6H2O and 1.3g of AlCl '6H O in75 ml of water. The reduction temperature was 330C.

EXAMPLE 6 Example 5 was repeated except that the reduction was carriedout at 370C.

EXAMPLE 7 The chloride solution was composed of 7.9g of FeCl .nl-l O,7.9g of CoCl hH O, 0.4g of NiCl .6l-I O, l.6g of AlCl .6l-l O and 75 mlof water. The reduction temperature was 300C.

EXAMPLE 8 Example 7 was repeated except that the reduction temperaturewas 330C.

EXAMPLE 9 7.0g of FeCl .nl-l O, 6.9g of CoCl .6H O, 0.3g of NiCl bl-l oand 3.6g of AlCl .6H O were dissolved in 75 ml of water, and thereduction was carried out at 330C.

For comparison, two reference examples will be presented, in which no Aland only about 0.1 mole% of Al were used, respectively. The reductionwas carried out at 330C, and the remaining conditions were similar tothose in the above examples.

REFERENCE EXAMPLE 1 8.8g of FeCl .nl-l O, 8.7g of CoCl .6H O and 0.4g ofNiCl .6H O were dissolved in 75 ml of water.

REFERENCE EXAMPLE 2 The chloride solution was similar to that in Example1 except that AlCl .6l-l O was decreased to 0.02g.

Table of the experimental data Reduction Coercive Saturation Residual AlTern peraturc Force Magnetization Magnetization (m l PC) i /g! mu/g1 EX.l about 1 3H] 880 I9] 64 EX. 2 4 3l0 980 I80 67 EX. 3 4 330 )ll) I90 66EX 4 4 370 750 l 90 54 EX 5 7 330 900 190 63 EX. 6 7 370 800 190 S7 EX.7 8 300 I020 [45 54 EX. 8 8 330 930 I94 70 EX. 9 I8 330 960 I73 64 REF.l 0 330 700 200 REF 2 about 0.l 330 700 203 EXAMPLE 2 It will beapparent from the above Table that an Al- Example 1 was repeated exceptthat the chloride socontaining Fe-Co-Ni alloy powder prepared by aprolution was prepared by dissolving 8.5g of FeCl .nl-I O, 8.2g of CoCl.6H O, 0.3g of NiCl .6H O and 0.7g of AlCl .6l-l O in 75 ml of water.

cess of the invention shows a remarkable increase both in coercive forceand residual magnetization accompanied with only a slight decrease insaturation magneti- 4.5 zation as compared with analogous alloyscontaining no or only a trace of Al.

Such advantages of the inventiori are illustrated in the graphs of theaccompanying drawings to'gether with influences of the Al content in themixed crystal and the reduction temperature on the magnetic propertiesof the resulting alloy powders.

The graphs of FIGS. 1, 2, 3 and 4 show the effect of the reductiontemperature in a conventional process on the coercive force, saturationmagnetization, residual magnetization and the rectangular ratio(residual magnetization/saturation magnetization), respectively, of aFe-Co-Ni alloy powder. The abscissas represent the reductiontemperature, and the ordinates the relative values of the aboveproperties, in which 1.0 represents the value obtained through reductionat 330C.

Referring to FIG. 1, the coercive force increases as the reductiontemperature is decreased. If a coercive force about 1.2 times as largeas that attained through the reduction at 330C (which is commonlyemployed in conventional methods) is desired, the reduction tem peraturemust be lowered to about 280C. Such a low temperature, however, causesthe reduction reaction to proceed very slowly or not at all, and hencerequires an extremely long reaction time and/or the use of a catalyst.Besides, even if the desired increase in the coercive force is attainedby reduction at a temperature below 300C, both the saturation andresidual magnetization values decrease sharply at such a temperature asseen from FIGS. 2 and 3. FIG. 4 also indicates that a significantincrease in the coercive force is impossible when reduction is carriedout at about 300C.

FIGS. 5-8 show the effect of the Al introduction into a Fe-Co-Ni alloyhaving a composition substantially equal to that obtained by reductionof the oxalates of Example 2 at 330C. The abscissas represent the amountof the introduced Al in mole% of the total metals, and the ordinatesrepresent the relative values of the magnetic properties being scaled bythe same standard as in FIGS. 1-4.

As seen from FIG. 5, the coercive force increases re markably as the Alcontent is increased up to about 5 mole% and continues to slightlyincrease thereafter. Augmentation in the residual magnetization is alsoattained in a wide range of Al content, but a peak exists in FIG. 7 atabout 4 mole% Al content. Although the saturation magnetization in FIG.6 decreases gradually as Al is increased, the rate of drop is very low,and FIG. 8 shows that the rectangular ratio is increased in a mannersimilar to the coercive force in FIG. 5. To put these data together, itis apparent that the introduction of Al into a Fe-Co-Ni alloy powder inan amount from about 1 to about mole% of the total metals brings about asurprising improvement in the coercive force and residual magnetizationof the alloy with only an insignificant reduction of the saturationmagnetization. Even when only 1 mole% of Al was added as illustrated inExample 1, the coercive force and residual magnetization increased byabout 40 percent and about 30 percent, respectively, and the rectangularratio was increased by about 50 percent compared with a correspondingconventional alloy containing no Al, while decrease in the saturationmagnetization was limited to about 5 percent. When a great importance isattached to the saturation magnetization, the Al content is preferablylimited to about l0 mole% at the most.

FIGS. 9-16 show the effect of the reduction temperature in the processaccording to the invention, in which 6 the ordinates are scaled by. thesame standard as in FIGS. 1-4. FIGS. 9-12 and 13-17 stand for 4 mole% A]content and 7 mole%, respectively.

As seen from these Figures, the augmentation in the coercive force andresidual magnetization by the introduction of Al can be attained whenthe reduction of the crystalline oxalate is carried out at a temperaturefrom about 300to about 400C. When a maximum augmentation is wanted, thereduction temperature is preferably kept within a range between about 3l0and about 350C.

In addition to the above described merits of a process of the invention,the simplicity and ease of practical application should be noted asimportant advantages of the process.

What is claimed is:

1. A process for the preparation of an aluminum-containing magneticalloy powder, comprising iron and at least one of cobalt and nickel, theprocess comprising the steps of:

preparing an aqueous solution of water-soluble salts of the respectivealloy component metals and a water-soluble salt of aluminum, the amountof said salt of aluminum being such that the amount of aluminum ion insaid solution is between I and 20 mole of the total metal ions;

regulating pH of said solution to about 1.5;

preparing a solution of oxalic acid in a mixture of a major amount ofacetone and a minor amount of toluene;

mixing said aqueous solution and said solution of oxalic acid withstirring to precipitate a crystalline powder consisting of the oxalatesof said alloy component metals and aluminum; and

heating said crystalline powder in a hydrogen atmosphere at atemperature between 300 and 400C until said powder is reduced topowdered metal magnetic alloy. 2. A process according to claim 1,wherein said mixture consists of about 200 parts by volume of acetoneand about 5 parts by volume of toluene.

3. A process according to claim 2, wherein said solution of oxalic acidis prepared by dissolving about 25 parts by weight of oxalic acid as Cd- 0 211 0 in about parts by weight of said mixture.

4. A process according to claim 1, wherein said water-soluble salts ofthe respective alloy component metals and said water-soluble salt ofaluminum are chlorides.

5. A process according to claim 1, wherein said heating step iscontinued for about 6 hr.

6. A process according to claim 1 for the preparation of a magneticalloy powder of Fe, Co, Ni and Al, comprising the steps of:

preparing an aqueous solution of FeCl CoCl NiCl and AlCl the amount ofAlCl being such that the amount of aluminum ion in said solution isbetween 1 and 20 mole of the total metal ions;

regulating pH of said solution to about 1.5;

preparing a solution of oxalic acid in a mixture of about 200 parts byvolume of acetone and about 5 parts by volume of toluene by dissolvingabout 25 parts by weight of oxalic acid as C H O .2I-I O in about 165parts by weight of said mixture;

mixing said aqueous solution with said solution of oxalic acid withstirring to precipitate a crystalline powder of the oxalates of Fe, Co,Ni and Al; and

7 8 heating said crystalline powder in a stream of hydro- 8. A processaccording to claim 6, wherein said temgen gas at a temperature between300and 400C p r mr i between 3lOand 350C. for about 6 hr to form saidmagnetic alloy powder. 9- A magnetic metal alloy powder prepared accord-7. A process according to claim 6, wherein said ing to the Proms5 ofclaim amount of aluminum ion is between I and 10 mole of A magnet: metalpowder prepared accord' mg to the process of claim 8. the total metalIOIIS. 4r t UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO. 1 3,925,11 r DATED December 9, 1975 INVENTOR(S) Nobor'u TAKAHASHI etal It is certified that error appears in the above-rdentified patent andthat said Letters Patent are hereby corrected as shown below:

In the Heading of the patent, please insert as Item [30] the following:

--Foreign Application Priority Data l May 1973 Japan HEB- 49125 23August 1973 Japan 48-93883 Signed and Scaled this sixth D3) Of April1976 :UTHC. MASON C. MARSHALL DANN Heslmg ()jju'er (mmm'ssr'rmeruj'latenrs and Trademarks

1. A PROCESS FOR THE PREPARATION OF AN ALUMINUM-CONTAINING MAGNETICALLOY POWDER, COMPRISING IRON AND AT LEAST ONE OF COBALT AND NICKEL, THEPROCESS COMPRISING THE STEPS OF: PREPARING AN AQUEOUS SOLUTION OFWATER-SOLUBLE SALTS OF THE RESPECTIVE ALLOY COMPONENT METALS AND AWATER-SOLUBLE SALT OF ALUMINUM, THE AMOUNT OF SAID SALT OF ALUMINUMBEING SUCH THAT THE AMOUNT OF ALUMINUM ION IN SAID SOLUTION IS BETWEEN 1AND 20 MOLE % OF THE TOTAL METAL IONS, REGULATING PH OF SAID SOLUTION TOABOUT 1,5; PREPARING A SOLUTION OF OXALIC ACID IN A MIXTURE OF A MAJORAMOUNT OF ACETONE AND A MINOR AMOUNT OF TOLUENE, MIXING SAID AQUEOUSSOLUTION AND SAID SOLUTION OF OXALIC ACID WITH STIRRING TO PRECIPITATE ACRYSTALLINE POWDER CONSISTING OF THE OXALATES OF SAID ALLOY COMPONENTMETALS AND ALUMINUM, AND HEATING SAID CRYSTALLINE POWDER IN A HYDROGENATOMSPHERE AT A TEMPERATURE BETWEEN 300 AND 400*C UNTIL SAID POWDER ISREDUCED TO POWDERED METAL MAGNETIC ALLOY.
 2. A process according toclaim 1, wherein said mixture consists of about 200 parts by volume ofacetone and about 5 parts by volume of toluene.
 3. A process accordingto claim 2, wherein said solution of oxalic acid is prepared bydissolving about 25 parts by weight of oxalic acid as C2H2O4.2H2O inabout 165 parts by weight of said mixture.
 4. A process according toclaim 1, wherein said water-soluble salts of the respective alloycomponent metals and said wAter-soluble salt of aluminum are chlorides.5. A process according to claim 1, wherein said heating step iscontinued for about 6 hr.
 6. A process according to claim 1 for thepreparation of a magnetic alloy powder of Fe, Co, Ni and Al, comprisingthe steps of: preparing an aqueous solution of FeCl2, CoCl2, NiCl2 andAlCl3, the amount of AlCl3 being such that the amount of aluminum ion insaid solution is between 1 and 20 mole % of the total metal ions;regulating pH of said solution to about 1.5; preparing a solution ofoxalic acid in a mixture of about 200 parts by volume of acetone andabout 5 parts by volume of toluene by dissolving about 25 parts byweight of oxalic acid as C2H2O4.2H2O in about 165 parts by weight ofsaid mixture; mixing said aqueous solution with said solution of oxalicacid with stirring to precipitate a crystalline powder of the oxalatesof Fe, Co, Ni and Al; and heating said crystalline powder in a stream ofhydrogen gas at a temperature between 300*and 400*C for about 6 hr toform said magnetic alloy powder.
 7. A process according to claim 6,wherein said amount of aluminum ion is between 1 and 10 mole % of thetotal metal ions.
 8. A process according to claim 6, wherein saidtemperature is between 310*and 350*C.
 9. A magnetic metal alloy powderprepared according to the process of claim
 1. 10. A magnetic metal alloypowder prepared according to the process of claim 8.